4.x: Improve Timer() scheduler handling

[akarnokd]

• Use a field _index to store the value to be emitted between schedule calls.
• Use stateful Schedule calls where appropriate.

[danielcweber]

Good without volatile ?

[akarnokd]

Yes, the schedulers won't execute Tick and Tock concurrently.

[akarnokd]

There is a ForwardOnNext in both Tick and Tock, which has always required those methods to run non-concurrently with themselves and the schedulers ensure that by default. The fact that they now take the value of a field is irrelevant because reading/writing that field followed by the ForwardOnNext call still has to be non-concurrent.